Oxidative homodimerization of substituted olefins by DDQ visible light photocatalysis

Abstract

A metal-free approach for the synthesis of buta-1,3-dienes from olefins using visible-light and catalytic amounts of DDQ is disclosed.

Excited‐State 2,3‐Dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ*) Initiated Organic Synthetic Transformations under Visible‐Light Irradiation

“…[26] In 2018, we reported a DDQ-photocatalyzed approach for the oxidative homo-coupling of a variety of olefins (5) to the corresponding symmetrical buta-1,3-dienes ( 6) under blue LED irradiation (Scheme 4). [27] The control experiments without either light or DDQ did not give any conversion to the product. Benzoquinone, anthraquinone and p-chloranil were studied as alternative catalysts in place of DDQ, but none of them gave better results than DDQ.…”

“…Nevertheless, oxidative dimerization was not observed for 1-phenylpropene (C 6 H 5 À CH=CHÀ CH 3 ), 2-vinylpyridine (C 5 H 4 NÀ CH=CH 2 ), and allylbenzene (C 6 H 5 À CH 2 À CH=CH 2 ) due to their poor stability under the employed reaction conditions. [27] A plausible mechanism for the DDQ-photocatalyzed formation of buta-1,3-dienes (6) from olefins (5) under visible-light irradiation is shown in Scheme 5. [27] Upon irradiation by visiblelight, DDQ is excited into its triplet state (i. e., 3 DDQ*), which oxidizes suitable olefinic-double bonds to the corresponding radical cations (7).…”

“…[27] A plausible mechanism for the DDQ-photocatalyzed formation of buta-1,3-dienes (6) from olefins (5) under visible-light irradiation is shown in Scheme 5. [27] Upon irradiation by visiblelight, DDQ is excited into its triplet state (i. e., 3 DDQ*), which oxidizes suitable olefinic-double bonds to the corresponding radical cations (7). Radical-addition reaction of radical cation intermediate 7 to another olefin yields a new radical-cation intermediate 8, which is then transformed into radical intermediate 9 by proton abstraction by [DDQ] * À .…”

“…In this reaction, DDQ was regenerated from DDQH 2 by oxidation using tert-butyl nitrite and molecular oxygen. [27]…”

Photocatalytic Homocoupling Transformations

“…Template for SYNTHESIS © Thieme Stuttgart • New York 2021-05-22 page 7 of 21 with a direct oxidative radical coupling followed by proton and hydrogen atom elimination (Scheme 14). 46 The recovery of DDQ was achieved by oxidation with t-butyl nitrite.…”